# A Piezoelectric-Actuated Variable Stiffness Miniature Rotary Joint

## TL;DR

This paper introduces a small, adaptable joint using piezoelectric materials to adjust stiffness quickly and safely in complex environments.

## Contribution

The novel contribution is a piezoelectric-actuated miniature rotary joint with real-time stiffness adjustment and stress monitoring.

## Key findings

- The joint's stiffness can be adjusted through piezoelectric actuation and friction control.
- A torque sensor based on strain monitoring enables real-time stress state tracking.
- Simulations and experiments confirm the feasibility of the stiffness adjustment mechanism.

## Abstract

With the acceleration of industrialization, deformable mechanisms that can adapt to complex environments have gained widespread applications. Joints serve as carriers for transmitting forces and motions between components, and their stiffness significantly influences the static and dynamic characteristics of deformable mechanisms. A variable stiffness joint is crucial for ensuring the safety and reliability of the system, as well as for enhancing environmental adaptability. However, existing variable stiffness joints fail to meet the requirements for miniaturization, lightweight construction, and fast response. This paper proposes a piezoelectric-actuated variable stiffness miniature rotary joint featuring a compact structure, monitorable loading state, and rapid response. Given that the piezoelectric stack expands and contracts when energized, this paper proposes a transmission principle for stiffness adjustment by varying the pressure and friction between active and passive components. This joint utilizes a flexible hinge mechanism for displacement amplification and incorporates a torque sensor based on strain monitoring. A static model is developed based on piezoelectric equations and displacement amplification characteristics, and simulations confirm the feasibility of the stiffness adjustment scheme. The mechanical characteristics of various flexible hinge structures are analyzed, and the effects of piezoelectric actuation capability and external load on stiffness adjustment are examined. The experimental results demonstrate that the joint can adjust stiffness, and the sensor is calibrated using the least squares algorithm to monitor the stress state of the joint in real time.